Five airlines which fly from Nairobi to London have been warned not to carry the four men, even if they are in transit to destinations outside Britain.

The move comes less than a week before Kenya’s presidential and parliamentary elections, due on Thursday, and sends a clear message that Mr Kibaki’s promise to stamp out corruption has failed.

“Britain has long had in place a policy of using visa bans against the corrupt in Kenya,” said Adam Wood, the High Commissioner to Kenya.

“I can confirm that airlines have been alerted that four individuals will not be allowed entry to the UK, but I am not willing to discuss the details of who they are.”

With Kenya on a knife-edge ahead of the December 27 ballot, the timing of the latest bans is likely to be interpreted by voters as a clear signal of Britain’s anger that Mr Kibaki’s pledge to root out corruption has turned out to be hollow.

Diplomatic sources stressed that the decision had ‘nothing to do with Kenyan domestic politics’ and had come after ‘extensive legal and ministerial consultations’ in Whitehall.

Mr Kibaki, 76, won a landslide with 63 percent of the vote in 2002 on a strong anti-graft ticket, after 24 corruption-tainted years under Daniel Arap Moi.

But voter anger at his failure to stamp out corruption has seen him trail his main challenger, Raila Odinga, 62, in all but one opinion poll since September. Mr Mwiraria, 69, the former finance minister, resigned from Mr Kibaki’s government in 2006 after he was named in connection with the notorious GBP100 million Anglo-Leasing scandal.

He was reinstated within months and has faced no charges since. He denies any involvement. Mr Biwott, 66, is one of Kenya’s wealthiest men, a close ally of Mr Moi and one-time chairman of KANU, Mr Moi’s party which ruled Kenya for 40 years.

He was implicated in the Moi-era Goldenberg scandal, when more than GBP500 million of central bank money was siphoned out of the country under bogus gold and diamond export schemes. He denies any corrupt involvement. Neither man could immediatly be reached for comment.

Chris Murungaru, former national security minister and a close ally of Mr Mwiraria’s, was the first minister to be banned from travel to Britain more than a year ago.